Microwave transmission on line-of-sight paths occasionally experiences severe fading associated with multiple transmission paths of differing propagation delays. An expression for the envelope delay distortion caused by two such paths is presented. Assuming that envelope delay distortions comparable to the symbol length of a digital signal could be intolerable, an expression is derived for critical pairvalues of fade depth and path delay for a digital signal transmitted over a two-path medium. Good agreement of the principle was obtained with laboratory measurements made on 4- and 8-level systems. Curves are given showing the relationship between critical fade depths and relative delays, and estimates are made of channel outages based on assumed distributions of fades and time delays. The outage times are extemely sensitive to hop length, and exceed a commonly used objective for hops mote than 25 mi long.